Tooth mineral is composed predominantly of calcium hydroxyapatite, Ca10(PO4)6(OH)2, which may be partially substituted with anions such as carbonate or fluoride, and cations such as zinc or magnesium. Tooth mineral may also contain non-apatitic mineral phases such as octacalcium phosphate and calcium carbonate.
Tooth loss may occur as a result of dental caries, which is a multifactorial disease where bacterial acids such as lactic acid produce sub-surface demineralisation that does not fully remineralise, resulting in progressive tissue loss and eventually cavity formation. The presence of a plaque biofilm is a prerequisite for dental caries, and acidogenic bacteria such as Streptococcus mutans may become pathogenic when levels of easily fermentable carbohydrate, such as sucrose, are elevated for extended periods of time.
Even in the absence of disease, loss of dental hard tissues can occur as a result of acid erosion and/or physical tooth wear; these processes are believed to act synergistically. Exposure of the dental hard tissues to acid causes demineralisation, resulting in surface softening and a decrease in mineral density. Under normal physiological conditions, demineralised tissues self-repair through the remineralising effects of saliva. Saliva is supersaturated with respect to calcium and phosphate, and in healthy individuals saliva secretion serves to wash out the acid challenge, and raises the pH so as to alter the equilibrium in favour of mineral deposition.
Dental erosion (i.e. acid erosion or acid wear) is a surface phenomenon that involves demineralisation, and ultimately complete dissolution of the tooth surface by acids that are not of bacterial origin. Most commonly the acid will be of dietary origin, such as citric acid from fruit or carbonated drinks, phosphoric acid from cola drinks and acetic acid such as from vinaigrette. Dental erosion may also be caused by repeated contact with hydrochloric acid (HCl) produced in the stomach, which may enter the oral cavity through an involuntary response such as gastroesophageal reflux, or through an induced response as may be encountered in sufferers of bulimia.
Tooth wear (ie physical tooth wear) is caused by attrition and/or abrasion. Attrition occurs when tooth surfaces rub against each other, a form of two-body wear. An often dramatic example is that observed in subjects with bruxism, a grinding habit where the applied forces are high, and is characterised by accelerated wear, particularly on the occlusal surfaces. Abrasion typically occurs as a result of three-body wear and the most common example is that associated with brushing with a toothpaste. In the case of fully mineralised enamel, levels of wear caused by commercially available toothpastes are minimal and of little or no clinical consequence. However, if enamel has been demineralised and softened by exposure to an erosive challenge, the enamel becomes more susceptible to tooth wear. Dentine is much softer than enamel and consequently is more susceptible to wear. Subjects with exposed dentine should avoid the use of highly abrasive toothpastes, such as those based on alumina. Again, softening of dentine by an erosive challenge will increase susceptibility of the tissue to wear.
EP-A-691124 (Sara Lee) describes oral care products comprising a copolymer of N-vinylpyrrolidone and acrylic acid, said to result in enhanced enamel fluoride uptake in the tooth enamel. On page 11 of Sara Lee the relative enamel fluoride uptake efficacy of various formulations containing different thickening agents, including a combination of xanthan gum and a carbopol is described.
French Patent No. 2755010 (Sara Lee) describes an oral care product containing fluoride comprising a combination of xanthan gum and a carboxylated vinyl polymer. Sara Lee specifically claim the use of this combination to enhance the efficacy of the fluoride. There is no specific disclosure of an oral care product containing any other thickening agents, although it is suggested that an oral care product such as a toothpaste may comprise various thickening agents including hectorite, carrageenan, gum tragacanth, starch, polyvinylpyrrolidone, various hydroxyalkyl celluloses, carboxymethylcellulose sodium and colloidal silica.
WO 08/068,323 describes a post-foaming dentifrice composition having a viscosity of at least 80,000 mPas comprising a combination of a first thickening agent which is xanthan gum, a second thickening agent which is carrageenan and/or a polyacrylic acid and a third thickening agent which is a thickening silica, together with a post-foaming agent to expand the composition to a foam. The Examples in WO 08/068,323 all show dentifrice compositions with a thickening system that comprises xanthan gum, carageenan and a thickening silica, and is incorporated by reference herein.
Contrary to the teaching in the above noted Sara Lee patent, the present applicants have found that the combination of xanthan gum and a polyacrylic acid (ie a carboxylated vinyl polymer) in an oral care product does not result in enhanced fluoride efficacy when compared with a corresponding product containing a polyacrylic acid in the absence of xanthan gum. It appears from Example 4 of the present specification that the presence of xanthan gum reduces the enhanced efficacy of oral care products containing a polyacrylic acid.
In addition the present applicants have found that oral care products containing the combination of xanthan gum, a polyacrylic acid and a thickening silica can suffer problems of poor rheology, such that the products are too runny and provide poor stand-up when dispensed onto a toothbrush.
The applicants have found that such problems can be overcome by using xanthan gum, carageenan gum, polyacrylic acid and a thickening silica as the thickening system in a dentifrice composition. This new formulation shows good stability plus excellent build and structure and therefore produces good stand-up when dispensed onto a toothbrush. The good build and structure that is produced proves particularly beneficial when preparing a striped dentifrice product.